Refractory compositions

ABSTRACT

Refractory insulating compositions which comprise mineral wool and ceramic fibre in admixture. Up to 60 percent by weight of the composition preferably can be mineral wool for low temperature applications; at higher temperatures a higher proportion of ceramic fibre is needed. The compositions may be mixed with carriers, binders and/or stiffeners.

United States Patent Ash Aug. 28, 1973 [5 REFRACTORY COMPOSITIONS3,249,568 5/l966. Reis 252/52 3,294,562 12/1966 Ca rio 106/64 [75]Invent: Ma|clm Dmmld 3,510,394 5/1970 Ca otte 106/40 R England [73]Assignee: Morganite Ceramic Fibres Limited,

Wirra], Engla d Primary ExaminerJames E. Poer Filed: g 1970Atl0rneyPlum1ey and Tyner [21] Appl. No.: 65,086

[30] Foreign Application Priority Data [57] ABSTRACT Aug. 21, 1969 GreatBritain 41,811/69 Refractory insulating compositions which comprise [52]U.S. CI. 106/55, 106/65 mineral W001 and mi fi re in admixture. Up t0-60[51] Int. Cl C04b 35/16 Percent y eight of the composition preferablycan be [58] Field of Search 106/99, 64, 40, 55; mineral wool for lowtemperature applications; at 252/62 higher temperatures a higherproportion of ceramic fibre is needed. The compositions may be mixedwith [56] References Cit d carriers, binders and/or stiffeners.

UNITED STATES PATENTS Scott 252/62 3 Claims, 3 Drawing FiguresREFRACTORY COMPOSITIONS This invention relates to refractory insulatingcompositions and in particular to compositions which comprise mineralwools.

A disadvantage of many refractory compositions is that they shrink whenheated, the shrinkage increasing as the temperature increases. For somepurposes shrinkage does not matter but often it is of cardinalimportance. Thus a refractory fibrous layer which is in contact with ahot surface will tend to curl if the temperature of the surface exceedsa certain value. This undesirable curling of the refractory layer iscaused by the fibres nearest the hot surface shrinking more than thefibres further away from the heat. Thus the maximum temperature at whicha particular refractory fibre material can conveniently be used isusually dictated by the shrinkage. For general purposes we regard ashrinkage of 5 percent as being the maximum tolerable, although for someapplications a figure of 3 percent is used.

Mineral wools, which are man-made fibres produced from molten minerals,have been used in place of asbestos, as they are serviceable at highertemperatures and do not involve the toxic hazard associated withasbestos. V

The term mineral wool, as used herein, includes slag wool and rock wool.Mineral wools can, for example, be produced by passing a stream ofmolten rock or slag in front of high pressure steam or air jets. Slag,which can be formed into such wools, is obtained as a by product fromvarious metallurgical processes and the composition of the slag willdepend on the particular process from which it has been obtained.

Rock wools are made from molten rocks or minerals, e.g., limestone,which may contain other rocks, e.g., shales. Clearly the preciseconstitution of a rock wool will depend on the starting mineral or rock.Wollastonite has been used; as has basalt, which is a dark colouredigneous rock which comprises plagioclase, feldspar and augite. Otherrocks have been used and these typically contain silica, alumina, lime,magnesia and iron oxide.

Irrespective of origin, the main chemical constituents of mineral woolsin percentages by weight are usually from 25 to 50% Si0,, 3 to 20% A1 20to 45% CaO and 3 to 18% MgO.

Mineral wools can be used at higher temperatures than asbestos, in arange of about 500 C. to 850 C.. Above 850 C. the shrinkage is above 5percent for most mineral wools and is, therefore, unacceptable.

If the working temperatures are greater than 850 C. it is usuallynecessary to use ceramic fibres instead which can be used in thetemperature range of 800 to 1,250" C..

Ceramic fibres are usually made from fused natural minerals such askaolins, bauxite, kyanite and certain fireclays or from mixtures ofalumina and silica and modifying agents. They can also be made fromviscous solutions. It seems that these fibres owe their highlyrefractory nature to a relatively high alumina content. Thus a typicalmineral wool as defined above, contains 3 to 20% alumina, whereasceramic fibres usually contain not less than 40% alumina. Ceramic fibrescan be produced having very reproducible properties as they are madefrom relatively pure starting materials. Ce-

ramic fibres are, however, very much more expensive than mineral wools.

Thus when working in a temperature range of 850 to 1,250 C. it haspreviously been necessary to use expensive ceramic fibres refractorycompositions.

It is an object of this invention to increase the maximum temperaturesat which mineral wools can be used, without the shrinkage becomingunacceptably large.

We have now found that it is possible to use mineral wools atsubstantially higher temperatures consistent with acceptable shrinkageif a proportion of ceramic fibres are admixed with the mineral wool.

We have further found surprisingly that the result of adding ceramicfibres to mineral wool reduces the shrinkage tendency of the ceramicfibre/mineral wool mixture to a greater extent than would be expected.

Accordingly the invention provides a refractory insulating compositionwhich comprises an admixture of mineral wool and ceramic fibre. Theinvention also provides articles which have been made from thesecompositions.

Another aspect of the invention is the provision of refractorycompositions which at a given working temperature exhibit a lowershrinkage than a pure mineral wool.

The compositions may contain additional materials, e.g., carriers,stiffeners and/or binders which may be in fibrous, powder or granularform. Examples of such additional materials are cements, raw or calcinedclays and alumina.

Although in a composition it is the individual fibres which shrink, whenthey are bonded into a composition material the whole article willshrink. One would, therefore, expect that a composition of mineral wooland ceramic fibre would have a shrinkage which depended on the relativeproportions of these two ingredients. For example at a giventemperature, e.g., 1,000 C., a particular mineral wool might have alarge (and practically quite unacceptable) shrinkage of 10 percentwhereas the shrinkage of ceramic fibre is only 1 percent. One mighttherefore reasonably expect that the shrinkage of a compositioncontaining 10 percent ceramic fibre and percent mineral wool should have10 percent of the properties of the ceramic fibre and 90 percent of theproperties of the mineral wool. This would lead one to expect an overallshrinkage of 9.1 percent. Conversely if 10 percent of the mixture weremineral wool one would expect a shrinkage of about 1.9 percent. However,this is not found to be the case, as the presence of relatively smallquantities of ceramic fibre in mineral wool compositionsdisproportionately reduces the overall shrinkage tendency and henceextends the maximum temperature at which mineral wool compositions canbe used.

Thus we have found that up to 60 percent of the composition can bemineral wool without the shrinkage properties of the ceramic fibrescomposition being seriously affected. This also means, of course, thatthe addition of the mineral wool does not reduce significantly themaximum temperature at which the ceramic fibre compositions can be used.

This is illustrated in the accompanying drawings in which FIGS. 1 and 2show the shrinkage curves for mineral wool/ceramic fibre compositions attwo different temperatures. Each graph shows two different compositionsusingdifferent commercially available mineral wools, Rocksil andStillite, which are described more fully below. It can be seen from FIG.1 that at 871 C. the shrinkage of the pure ceramic fibre is about 0.8percent whereas, the shrinkage of pure Stillite is about 8 percent. Itcan be seen from the curve that the presence of only 30 percent ofceramic fibres reduces the shrinkage to almost 2 percent. FIG.'2 shows avery similar pattern at a lower temperature (817 C.).

Although at 817 C. pure Rocksil has an acceptable shrinkage, FIG. 2illustrates the possibility of reducing the shrinkage further by theaddition of relatively small amounts of ceramic fibre.

FIG. 3 shows the variation of the percentage shrinking of variousceramic fibre/mineral wool compositions at different temperatures. Asthe figure of 5 percent shrinkage is taken as the maximum allowable itwill be seen that pure Stillite cannot be used above 800 C. and pureRocksil above 850 C., whereas the addition of only 30 percent of ceramicfibre leads to a very acceptable shrinkage at temperatures as high asl,l50 C..

The measurement of the shrinkages given in the graphs are determined bymaking small bricks which are made predominantly of the mineral fibretogether with an organic or, for example, a cement binder. The brick ismeasured and then heated to a given temperature and maintained at thattemperature until maximum shrinkage has occurred and is then cooledagain. The size of the brick before heat treatment and after heattreatment at room temperature are compared. Thus percentage linearshrinkage can then be easily calculated;

Clearly the amount of ceramic fibre added to a mineral wool compositionwill depend on the particular application for which it is intended andin particular on the maximum tolerable shrinkage and on the maximumtemperature at which the refractory composition is likely to be used.The amount of ceramic fibre which is added will also depend on costconsiderations. At the present time the cost of ceramic fibres isapproximately six to 12 times the costs of mineral wool and hence it isusually desirable to use as small a proportion as possible of theceramic fibre. Usually it is not necessary to use more than 50 percentby weight of the ceramic fibre and we have found that compositionscontaining 30 percent by weight of ceramic fibre are satisfactory formany applications.

It will be seen that one advantage of the invention is that it is nowpossible to use mineral wools at much higher operating temperatureswithout the drawback of an unacceptably high shrinkage. Anotheradvantage of the invention is that it makes it possible to producecheaper ceramic fibre compositions because the addition of relativelylarge amounts of the cheaper mineral wooldo not impair the properties ofthe compositions too greatly.

The refractory compositions according to the invention can be preparedby mixing the ceramic fibres with the mineral wools. For someapplications it may be desirable to chop the fibre tufts before they aremixed. The refractory compositions can, for example, be used to prepareby air or water deposition or vacuum forming, fibrous articles such asblankets, felts, blocks,

woven or unwoven textiles. The fibres may be used in the bulk form inwhich they are produced and processed into blankets or other desiredshapes with no additional ingredients.

The following Examples gives representative mineral wool/ceramic fibrecompositions according to the invention.

EXAMPLE 1 The following composition was prepared 69.5% by weight Mineralwool Rocksil 29.5% by weight Ceramic fibre Triton Kaowool 1% by weightMethofas P.M. 1500 The composition was thoroughly mixed and was thenmixed with water and formed into blocks and boards. The products werefound to be useful at temperatures up to 1150 C. and had a shrinkage ofless than 3% at this temperature. The shrinkage curve of bricks madefrom this composition is shown in FIG. 3 (designated 70% Rocksil).

EXAMPLE 2 The following composition was prepared 59.5% by weight Mineralwool Stillite 39.5% by weight Ceramic fibre Triton Kaowool 1% by weightMethofas P.M. 1500 The composition was prepared and used in the same wayas in Example 1. Products made from this composition had an even smallershrinkage (see FIG. 3; curve designated 60% Rocksill or Stillite).

EXAMPLE 3 The following composition was prepared 25% by weight Mineralwool Stillite 10% by weight Ceramic fibre Triton Kaowool 30% by weightcolloidal silica Syton 30X 5% by weight micronised alumina MA 130 30% byweight water The composition was thoroughly mixed and then formed intoblocks and boards which were allowed to set. The products were found tobe useful at temperatures of up to 1150 C., having a shrinkage below 5%.

Rocksil is a rock wool which is manufactured and sold by the CapeAsbestos Co.. It is thought to be produced from a dolomite type oflimestone and a siliceous fireclay.

Stillite is a slag wool and is produced by Stillite Products Ltd..

The ceramic fibre used was Triton Kaowool which is available fromMorganite Ceramic Fibres Ltd., and which is produced from kaolin, anaturally occurring alumina silicate fireclay of high purity.

The Syton 30X colloidal silica is manufactured by Monsanto ChemicalsLtd., the micronised alumina by B.A. Chemicals Ltd., and the Methofas PM1,500

boards, special shapes, paper products; by dry or wet which is a methylhydroxypropyl cellulose manufactured by Imperial Chemical IndustriesLtd..

What I claim is:

l. A refractory insulating composition consisting essentially of anadmixture of mineral wool containing 320% alumina and ceramic fibercontaining not less than 40% alumina said ceramic fiber comprising 10%to about 50% by weight based on the weight of ceramic fiber and mineralwool.

2. The composition of claim 1 comprising up to about 60% by weight ofmineral wool.

3. The composition of claim 1 comprising about 10 to about 30% by weightof ceramic fibre.

2. The composition of claim 1 comprising up to about 60% by weight ofmineral wool.
 3. The composition of claim 1 comprising about 10 to about30% by weight of ceramic fibre.